Rubber compositions for components of tires are often comprised of diene-based elastomers which contain filler reinforcement composed of a combination of rubber reinforcing carbon black and precipitated silica. Such precipitated silica is an amorphous synthetic silica which contains hydroxyl groups on its surface.
The precipitated silica with its included hydroxyl groups is hydrophilic in nature and the presence of the included hydroxyl groups causes it to be significantly more polar than less polar diene-based elastomers contained in the rubber composition. The high polarity of the hydrophilic precipitated silica particles causes the silica particles to be more attractive to other hydrophilic precipitated silica particles (causes a high silica-silica attraction) contained in a rubber composition and less attractive to the less polar diene-based elastomers (less silica-elastomer attraction) in the rubber composition.
Therefore, creating an efficient dispersion of the highly polar hydrophilic precipitated silica within the diene-based elastomer-containing rubber composition can be difficult without extensive high shear mixing of the rubber composition. Such difficulty is well known to those having skill in such art.
In practice, in order to promote reinforcement of the diene-based elastomer in the rubber composition by the precipitated silica, a silica coupler is normally added to the rubber composition. The silica coupler contains a moiety (e.g. an alkoxysilane) reactive with the hydroxyl groups on the precipitated silica and another different moiety (e.g. a polysulfide) interactive with the diene-based elastomer(s) contained in the rubber composition. The reaction of such silica coupler with hydroxyl groups on the precipitated silica during the mixing of the rubber composition, therefore an in situ reaction within the rubber composition, reduces the content of the of hydroxyl groups on the silica with a portion of unreacted hydroxyl groups remaining, to thereby render the precipitated silica less polar in nature and therefore more dispersible within the rubber composition.
Sulfur with various combinations of sulfur cure accelerators may be used to cure the precipitated silica-containing, diene rubber based rubber composition where the accelerators are typically composed of a primary accelerator together with a more active secondary accelerator to aid in initiating and promoting the sulfur vulcanization. The rate of sulfur cure of the rubber composition promoted by the sulfur cure accelerators is important in a sense that it must be sufficient to promote curing of the rubber composition within a suitable period of time but must not be too rapid in order to avoid what is sometimes referred to as an unwanted “scorching”, or “pre-curing” of the rubber composition.
Representative of primary sulfur cure accelerators are sulfenamide primary sulfur cure accelerators. Representative of such sulfenamide primary sulfur cure accelerators are, for example, N-cyclohexyl benzothiozole-2-sulfonamide, N-dicyclohexyl benzothiazole-2-sulfenamide and N-tert-butyl benzothiazole-2-sulfenamide.
However, such in situ modified precipitated silica retains a degree of its polarity sufficient to attract a portion of later added sulfur cure accelerators to thereby reduce availability of the sulfur cure accelerators and associated rate of cure promoted by the accelerators in the rubber composition. To counteract such reduction in rate of cure activity, a more active diphenylguanidine secondary accelerator is added together with the primary sulfenamide primary accelerator to thereby increase the reduced rate of cure of the rubber composition.
Alternatively, the polar hydrophilic precipitated silica may be pre-treated with a silica coupling agent prior to its introduction into the rubber composition to make it more hydrophobic, to thereby reduce its polarity, making it less polar in nature and to render it more compatible with and thereby more readily dispersible within the less polar diene-based elastomer-containing rubber composition.
It is envisioned that such pre-treatment of the precipitated silica prior to addition to the rubber composition instead of blending silica coupling agent with the precipitated silica within a rubber composition provides a more efficient reaction of the coupling agent with hydroxyl groups on the precipitated silica. Therefore a greater reduction of the concentration of hydroxyl groups on the precipitated silica is provided accompanied by a greater reduction in the polarity of the precipitated silica.
With the greater reduction in polarity of the pre-treated precipitated silica, less sulfur cure accelerator is thereby absorbed onto the pre-treated precipitated silica within the rubber composition during the rubber mixing process and more sulfur cure accelerator remains available for sulfur curing the rubber composition which ultimately promotes both a faster rate of sulfur curing of the rubber composition as well as an adverse increase in the sulfur cure density formation for the rubber composition. In one aspect, such increase of the sulfur cure activity may undesirably promote an excessively faster cure rate for the rubber composition at a lower temperature to an extent that the rubber may partially pre-sulfur cure during the mixing of the rubber composition and prior to a molding and intended curing of the rubber composition. Such undesirable pre-sulfur curing of the rubber composition during its mixing process is sometimes referred to as “scorching”. Scorching of the rubber composition during mixing is generally undesirable as is known by those having skill in such art. The increase in sulfur cure density may adversely affect (e.g. reduce) the cured rubber's elongation to break and adversely reduce its tear resistance.
To reduce promotion of unwanted scorching (pre-sulfur curing) of the rubber. composition during its mixing and increase in sulfur crosslink density, it may be readily thought of to simply eliminate presence of the secondary accelerator (e.g. the diphenylguanidine). However, such elimination of the diphenylguanidine secondary accelerator may adversely cause a reduction of various cured rubber properties.
Accordingly, a challenge is presented to provide a sulfur vulcanization system comprised of sulfur and sulfur cure accelerator(s), which includes a sulfenamide primary cure accelerator, where a rubber composition contains a precipitated silica pre-treated with a silica coupler (pre-treated prior to its addition to the rubber composition).
Therefore, it is desired to evaluate an inclusion of a crosslinking agent for a rubber composition comprised of at least one diene-based elastomer and precipitated silica pre-treated with silica coupler where the sulfur cure package is comprised of sulfur and sulfenamide primary cure accelerator. Such crosslinking agent for this evaluation is 1,6-bis (N,N′-dibenzylthiocarbamyldithio)-hexane.
In the description of this invention, the terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The terms “cured” and “vulcanized” may be used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of an ingredient per 100 parts by weight of rubber in a rubber composition.